1. Field of the Invention
The present invention relates to a refrigerant compressor, and more particularly, to a wobble plate type compressor with a variable displacement mechanism suitable for use in an automotive air conditioning system.
2. Description of the Prior Art
It has been recognized that it is desirable to provide a wobble plate type piston compressor with a displacement or capacity adjusting mechanism to control the compression ratio in response to demand. As disclosed in U.S. Pat. No. 4,428,718, the compression ratio may be controlled by changing the slant angle of the sloping surface of a slant plate in response to operation of a valve control mechanism. The slant angle of the slant plate is adjusted to maintain a constant suction pressure in response to a change in the heat load of the evaporator of an external circuit including the compressor or a change in rotation speed of the compressor.
In an air conditioning system, a pipe member connects the outlet of an evaporator to the suction chamber of the compressor. Accordingly, a pressure loss occurs between the suction chamber and the outlet of the evaporator which is directly proportional to the "suction flow rate" therebetween as shown in FIG. 5. As a result, as shown by the dash line in FIG. 4, when the capacity of the compressor is adjusted to maintain a constant suction chamber pressure in response to a change in the heat load of the evaporator or the rotation speed of the compressor, the pressure at the evaporator outlet increases as well. This increase in the evaporator outlet pressure results in an undesirable decrease in the heat exchanging ability of the evaporator.
U.S. Pat. No. 4,428,718 discloses a valve control mechanism to eliminate this problem. The valve control mechanism includes a device which senses the discharge pressure of the compressor and in response thereto, the valve element is shifted to maintain a constant pressure at the evaporator outlet portion. That is, the valve control mechanism makes use of the fact that the discharge pressure of the compressor is roughly directly proportional to the suction flow rate.
However, the relationship between the discharge pressure and the suction flow rate is not constant in every air conditioning system. Furthermore, the discharge pressure varies greatly in response to the velocity of air passing through the condensor. Accordingly, in an automotive air conditioning system in which the wind velocity varies greatly in response to the speed of the vehicle, the relationship is indefinite and unreliable. Therefore, the system is not sufficiently effective in preventing the undesirable increase in pressure at the evaporator outlet.